1. Field of the Invention
The present invention relates to a group of winding electrodes, and more particularly, to a novel separator with high durability and capacity, and low internal pressure for use in a cylindrical cell or a condenser.
2. Description of the Related Art
A group of winding electrodes, such as a cylindrical cell or a condenser, has a separator to prevent a short-circuit between the cathode and the anode. The present invention relates to a technology for a group of winding electrodes having a cathode, anode and separator. A cylindrical cell, especially a cylindrical nickel-hydrogen cell, is described as an example.
More and more portable electronic devices, such as a camera, a camcorder, a portable CDP, a radio, a cassette, a notebook computer, a pager and a cellular phone, etc., require a cell having higher capacity and longer durability.
In general, a cell is a device that converts chemical energy into electric energy by means of contact potential difference and many kinds of such cells are conventionally known to those skilled in the art. Electrochemical cells and batteries are technologically identified as nonchargeable primary cells, rechargeable secondary cells, fuel cells that convert combustion heat into electric energy or solar cells that convert light energy into electric energy. Electrochemical cells and batteries are classified by the composition of electrolyte and the shape of the cells and batteries. The composition of electrolyte is alkaline, solid or nonaqueous cells and the shape is cylindrical, button or coin type.
In these kinds of cells, a cylindrical cell (jelly-roll type) discharges current and is composed of a cathode, an anode, a separator to prevent a short-circuit between the cathode and the anode, an electrolyte, a positive terminal, and a negative terminal. The structure of a nickel-hydrogen cell is illustrated in FIG. 8. The cylindrical nickel-hydrogen cell is composed of a cathode (13) coated with Ni(OH).sub.2 as a positive active material, an anode (15) of hydrogenated alloy coated with a negative active material which is mainly composed of LaNi.sub.5, MmNi.sub.5, Ti--Fe or Ti--Ni alloy, a separator (17) which is made of a nonwoven fabric and a cellophane tape to prevent a short-circuit between the cathode (13) and the anode (15), a cap (19) as a positive terminal, a case (12) as a negative terminal a gasket (21), a safety vent (23), a cover plate (25), an insulating ring (27), and an insulating plate (29).
The process for fabricating a cylindrical nickel-hydrogen cell is as follows. First, the cathode is manufactured by coating a slurry positive active material on a metallic support and then drying and rolling the coated metallic support. The anode is manufactured by coating a slurry negative active material on a metallic support and then drying and rolling the coated metallic support. After that, a separator is laid between the cathode and the anode, and is wound. The wound assembly comprising the electrodes and separator is inserted into the can. After that, an electrolyte is poured into the can and a cap assembly is mounted in the mouth of the upper part.
A detailed description of the charge and the discharge reaction of the cylindrical nickel-hydrogen cell manufactured according to the above method is as follows.
A hydrogenated alloy is used as a negative active material, nickel hydroxide is used as a positive active material and potassium hydroxide (KOH) aqueous solution is used as an electrolyte. The hydrogenated alloy stores hydrogen ions produced by cleavage of water in the electrolyte during the charging process, and releases hydrogen ions into the electrolyte during the discharging process. The charge and discharge reactions are as follows. ##STR1##
In the above reactions, M is a hydrogenated alloy that can absorb and emit hydrogen ions, identified as an AB.sub.5 group that is made of rare earth elements or an AB.sub.2 group that is made of Ti, Zr, V, etc. According to the above reaction, a cell performs a charge and discharge more than hundreds of times.
In the process for fabricating the cylindrical nickel-hydrogen cell with the above function and structure, as shown in FIG. 1, a cathode (13) and an anode (15) are placed on the opposite sides of a separator (17) to wind by using a mandrel around the center of the winding axis (11). But the separator, used to prevent a short-circuit between the cathode and the anode, takes up a lot of space in the can, and decreases the internal space in the cell for the injection of electrolyte and, therefore, reduces the capacity of cell. In addition, when the cathode (13), the anode (15) and the separator (17) are wound in the above way, the separator can rip, thus allowing the cathode and anode to short.
As shown FIG. 2, a technique of using an additional separator (31) covering the initial part of the cathode (13) to the initial part of the anode (15) is developed to prevent short-circuiting due to cracking of the separator (17). Although short-circuiting may be prevented, the additional separator (31) reduces the capacity of the cell because of the additional space it occupies.
As shown FIG. 3, an alternative approach is known wherein an additional separator (41) is positioned to cover from the winding axis to the initial part of the cathode (13). This approach solves the problem of decreased capacity of a cell, however, this approach still has the problem that the capacity of a cell is not increased that much because of the limits in decreasing the volume of separator.
The foregoing defect occurs in all kinds of groups of winding electrodes including a cylindrical cell and a cylindrical condenser as well as the foregoing cylindrical nickel-hydrogen cell.